1. A stream represents flow of data from one place to another place.
  2. Input Stream:Which receive or read data coming from some other place.
  3. Output Stream:Which send or write data to some other place.

Note:All streams are represented by classes in a package and that is java.io package.

Stream Description
System.out It represents monitor(PrintStream object)
System.in It represents keyboard(InputStream object)
System.err It Represents PrintStream object,Which is by default represents monitor.

Difference between System.out and System.err?

System.out and System.err both represents the monitor by default,but System.out is used to display normal messages and results where as System.err is used to display error messages.